Dry developers used in the development of image forming apparatuses using electrophotographic technology may be categorized in the two-component developer in which toner and carrier consisting of ferrite powder, iron powder, glass beads, etc., are mixed, the magnetic one-component developer in which magnetic powder is contained in the toner particles, and the nonmagnetic one-component developer which does not utilize magnetic force. Toners used in these developers comprise binder resins and colorants as main components, and waxes for improving low-temperature fixability on recording sheets and releasing properties from fixing rollers, and charge control agents for imparting charge (positive charge or negative charge), etc., are added in addition to the main components. The above materials are mixed at a predetermined ratio, and then the mixture is subjected to processing such as melting and kneading processing, pulverizing processing, classifying processing, etc., so as to produce toner powder, and may then be surface-treated with external additives such as silica, titania, alumina, and various fine resin particles in order to control fluidity, charging property, cleaning property, storage stability of the toner, etc., and a developer is finally provided.
Recently, in the image forming apparatuses as described above, multifunctionalization, speeding up of copying to spread in the boundary region between printing machines and copying machines, down sizing, and energy saving (saving in electric power consumption) primarily for saving costs, have been desired. According to these desires, simplification and energy saving in fixing systems such as heat roller fixing systems are required, and the characteristics of the toner which is adapted to the fixing systems as described above and which is adapted to speeding up of copying and long life durability of the developer are required. Therefore, in the toner, coexistence of two contradictory characteristics is required, that is, superior fixability should be obtained even by using small amounts of energy, and stress resistance such as resistance to toner melt contamination on a charging blade, etc., should be improved.
In order to satisfy such requirements, styrene-acrylic resin has primarily been used as a binder resin for toner up until now. However, in toners containing styrene-acrylic resin, although fixability is easily improved, and there is a problem of stress resistance reduction that the toner is easily crushed by friction with sleeves, etc., because the strength of the resin itself lowers when the improved fixability is obtained. Thus, polyester resin is widely used in order to make up for the insufficient stress resistance. However, in the toners containing polyester resin, charging amount is greatly changed in low-temperature and low-humidity environments, and the thickness of a toner layer on a sleeve is increased with increase of the charging amount, and as a result, problems of the excessive developing amount and the generation of background fogging occur, and therefore, toners having satisfactory qualities are not obtained under the present circumstances.